1. Field of the Invention
This invention is concerned with a method and apparatus for nulling the direct coupled signals detected by the receiver coils of an induction logging tool.
2. Discussion of Related Art
Electromagnetic (induction) logging tools, used for measuring the electrical characteristics of formations penetrated by a borehole, are well known. Typically the tool consists of a sonde that is lowered into a borehole on the end of a supporting cable. An induction logging tool includes a transmitter coil and at least one and preferably a set of receivers that may include at least two spaced-apart receiver coils coaxially mounted relative to the transmitter coil but longitudinally separated therefrom. Powered by either a pulsed signal or a CW signal in the frequency range of 5-500 kilohertz (kHz), at a potential on the order of 100 volts, the transmitter radiates a magnetic field in the surrounding formation. The magnetic field induces eddy currents in the formation which are detected by one or more receivers. The magnitude of the induced potential, on the order of nanovolts, is a function of the formation conductivity.
Quantities of interest in certain applications are the amplitude and the phase of the received signals with respect to the current in the transmitter. In order to measure the signal caused by the eddy currents in the formation, it is necessary to cancel the large direct-coupled signal between transmitter and receiver coils.
A typical sonde is about 9 cm in diameter and 4 to 5 meters long. The sonde is constructed of non-magnetic material which also may be nonconductive such as fiberglass. In the simplest arrangement, a transmitter coil is provided at one end of the sonde. The receiver coils of a set may be 0.05 to 1.5 meters apart and separated from the transmitter coil by 0.1 to 3.0 meters.
The simplest configuration for an induction instrument is a two-coil system consisting of a single transmitter and a single receiver coil. In that configuration, the direct coupled signal, i.e. the signal present even in a non-conductive environment can exceed the signal sensitive to the presence of earth formations by four orders of magnitude. Typically, we wish to measure signals in formations characterized by a conductivity as low as 1 millisiemen/meter (mS/m). In the frequency range typically used, the direct coupled signal may range from 1 to 10 S/m. We are interested in the signal that is out of phase with respect to the direct coupled signal. That requires a phase measurement accuracy of 0.0006 degree. Furthermore, we are interested in the formation signal that is in phase with the direct coupled signal which desideratum requires an instrumental stability of one part in 100,000. Such precision is not presently achievable. For that reason, practical induction tools include at least one additional receiver coil that is wound in series with the first coil and positioned so that the direct coupled signal is approximate equal and opposite to the signal from the first coil, thereby to null the direct-coupled field. See for example, U.S. Pat. No. 4,873,488, issued Oct. 10, 1989 to T. D. Barber et al.
The direct-coupled field strength is very sensitive to the longitudinal position of the receiver coil with respect to the transmitter coil, being proportional to 1/d.sup.3 where d is the receiver-transmitter distance. Therefore, a very small longitudinal shift results in a relatively large change in the nulling effect, thus necessitating critical adjustment of the receiver-coil position. Customarily, the coil mounting arrangement includes means for spacing the coils using shims on the order of 0.0003 centimeter in thickness and thereafter locking the coil in place, all of which requires very tight manufacturing tolerances.
In more complex arrangements, such as when two transmitters are used or where an array of several sets of variously-spaced receiver coils are used, spacing of the various coils becomes very laborious since the adjustment of one shim may affect the adjustment of others.
There is a need for a less critical yet effective means for balancing the electrical outputs of the respective receiver coils of an array of one or more sets of receiver coils mounted in an induction logging tool.